how to connect output to drive multiple transistors?

Thread Starter

tpny

Joined May 6, 2012
220
If I want my output to drive two transistors, is (a) or (b) below better?

Rich (BB code):
a) output ---resistor-----transistor base1
                      |
                      |
                      -----transistor base2
 
or
 
b) output-----resistor-----transistor base1
         |
         |
         ---resistor-----transistor base2
 
Last edited:

Austin Clark

Joined Dec 28, 2011
412
b) is the proper choice.

a) will allow the transistor with the slightly lower base-emitter voltage to hog a majority of the base current and the other transistor may not turn on. Never do a)
Aye. Basically, the current capable of flowing through the resistor in example A will be split to both transistors, and unevenly at that. This is true for driving LEDs and other devices as well. Imagine trying to light a thousand LEDs this way, or driving a thousand transistors... The sum of current required will far exceed the maximum possible current capable of passing through the single series resistor given it's resistance and source voltage. As a consequence, you'll get a heavy voltage drop, and the resistor may even become overloaded.
 

Thread Starter

tpny

Joined May 6, 2012
220
assuming i'm using a resistor value in (a) that is the parallel equivalent of the 2 resistors in (b); and assuming both transistors are the same type. Are (a) and (b) equivalent?
 
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